1. Field of the Invention
This invention relates generally to an electric refrigerator or freezer and specifically to insulating walls therefor.
2. Description of the Related Art
It is previously known to use different types of insulation material for refrigerator and freezer cabinets in order to achieve as good insulation characteristics as possible for walls and doors of the cabinet. Usually foamed plastic materials having open as well as closed cell structures are used between inner and outer metal plates and/or plastic shells in the walls, but it has also been suggested to use different types of insulating powder materials.
In order to reduce the power consumption of the cabinets, it has also been suggested to use so called "vacuum panels" in the walls and the doors. See for instance EP 188806. When manufacturing these panels a powder or cellular material is surrounded by a diffusion-tight layer which is placed between the outer and the inner shell of the wall, after which the space containing the powder is evacuated and sealed. By means of this method it is, however, in industrial processes, difficult to reach sufficiently low pressures to maximize the insulation characteristics, since the evacuation process is very time consuming and is not well suited for mass fabrication. It should in this connection be mentioned that the time which is needed for evacuation to a pressure of about 1 mbar during the conditions mentioned above is about 15 hours, whereas the production time for a refrigerator is about 20 minutes. The evacuation time cannot be improved by using a pump with a higher capacity, since the evacuation time is determined by the narrow communication passages which are present in the powder or cellular material. Of course, there also is a risk that, during the life time of a refrigerator, which is 15-20 years, there will be a leakage through the diffusion-tight layer which means that the contribution which the vacuum gives to the insulation characteristics disappears.
It is also previously known, see U.S. Pat. No. 4,448,041, to use vacuum insulated wall elements for large mobile refrigerating chambers in which the wall elements are connected to a vacuum pump. However, these vacuum pumps are of a conventional type and hence relatively power demanding and expensive. Their use can, with regard to costs and energy consumption, only be suggested for the type of large equipment which is described in the above-mentioned publication.
Further, FR 2628179 describes hermetically sealed wall elements which, in a way which is not described in detail, are connected to some kind of vacuum source. The pressure which is created, 50-100 mbar, is rather high and within such an interval that it can not in any significant way contribute to increase the heat insulating characteristics.